U.S. Pat. No. 5,070,956 describes a hydraulic power assist steering system having conventional relatively rotatable spool and valve body elements coupled to a vehicle steerable wheel and a steering or hand wheel for regulation of a hydraulic steering assist boost pressure, a torsion bar creating a mechanical centering torque between the spool and valve body elements, and an integral electromagnetic mechanism which defines an additional coupling of variable resilience between the spool and valve body elements for adjusting driver steering effort required to produce a given level of power assist.
The integral electromagnetic mechanism comprises a stationary flux conducting element 136, rotary flux conducting elements 130, 132 supported for rotation with each of the spool and valve body elements and a stationary exciting coil disposed radially outside the rotary flux conducting elements and magnetically coupled thereto through the stationary flux conducting element to provide a flux path through the flux conducting elements and through air gaps therebetween. The rotary flux conducting elements each have an equal number of teeth projecting into the gap toward the other element to vary the reluctance of the air gap upon relative rotation. No permanent magnet is used.
When the exciting coil is energized with direct current, the teeth of each rotary element define electromagnetic poles which interact with the poles of the other element. The electromagnetic poles are oriented such that, when the spool and valve body elements are centered, with no torque in the torsion bar, the poles are radially aligned. This produces an attractive magnetic force between the poles and a positive magnetic centering torque when there is relative rotational displacement of the spool and valve body elements, which torque tends to restore the assembly to the centered position. The magnitude of the restoring torque depends on the magnitude of electric current provided through the exciting coil.
Structurally, one of the rotary elements is rotationally fixed to an inboard end of the spool element and the other, which circumferentially surrounds the one, is fixed to a pinion gear coupled to an inboard end of the torsion bar and the valve body element. This structure would provide four significant air gaps in the magnetic flux circuit were it not for the stationary flux conducting element, which reduces the number of significant air gaps to three. However, the stationary flux conducting element also provides a parasitic flux path between extension 140 and rotary flux conducting element 132, which path bypasses rotary flux conducting element 130 and which may significantly reduce the torque. The exciting coil is disposed circumferentially around the outermost of the rotary elements; and this increases the diameter of the housing, which must fit in a crowded engine compartment. There is an approximately 1:1 ratio between the circumferential width of the teeth and the spacing between the teeth on the rotary elements.
An improvement of the apparatus shown in U.S. Pat. No. 5,070,956 is found in the U.S. Patent application U.S. Ser. No. 08/687,077, U.S. Pat. No. 5,738,182, filed Jul. 17, 1996 by Birsching et al and assigned to the assignee of this application. In the improved apparatus, the external diameter of the apparatus is reduced by locating the relatively rotatable teeth of the variable reluctance type torque adjustment apparatus axially beside the exciting coil; and the smaller diameter housing fits more easily into a crowded engine compartment. The inner set of teeth is formed on an axial extension of the valve body projecting through the exciting coil; and the outer set of teeth is formed on a magnetic portion of an outer pole member having a non-magnetic portion mounted on the spool shaft. This provides three air gaps without requiring the stationary flux conducting element and thus eliminates a part and a parasitic flux path to decrease cost and increase peak restoring torque.
In this improved apparatus, the extension of the valve body bearing the inner teeth comprises sintered powdered iron, a material providing considerable manufacturing advantages for a part with a multiple toothed configuration but providing somewhat lower magnetization for a given magnetizing coil current than some other magnetic materials. The lower magnetization limits the maximum magnetic flux in the magnetic circuit and therefore the maximum magnetic field strength B produced in the air gap before saturation of the part and thus also limits the maximum torque output of the device for a given size. This problem is exaggerated in this particular part, since it passes through the coil, where the current generated H-field is highest, and is thus the part of the magnetic circuit closest to saturation. In addition, the cross-sectional area is small, due to the necessity of passing through the coil, and the flux path therein is also comparatively long. Each of these dimensional factors provides limits to the maximum magnetic flux achievable through the part. The use of available magnetic materials providing higher magnetization for the given magnetizing coil current, such as wrought steel, can increase output torque by as much as 33% but also tends to increase cost relative to the use of sintered powdered metal, due to the necessity of separately machining each tooth of the multi-tooth structure; while an increase in the cross-sectional area of the part increases both cost and package size.